1. Field of the Invention
The present invention relates to a method for etching films on semiconductor wafers for integrated circuits. More particularly, it relates to a method for improving etch uniformity during a wet etching process.
2. Description of the Related Arts
Integrated circuit fabrication processes that use reactive plasmas are commonplace in today's semiconductor production lines because of their potential for very high-accuracy transfer of resist patterns, i.e., anisotropic etching. By contrast, wet chemical etching results in isotropic etching, where both vertical and lateral etch rates are comparable. Nevertheless, wet etching is still the practical alternative for a high-throughput, flexible production process. With properly selected chemicals, etch reactions with the primary film are thermodynamically favored over reactions with the other films. The etch-rate ratio usually approaches infinity. Wet etches are especially suitable for blanket etches of polysilicon, oxide, nitride, and metal. Recently, there has been a resurgence of wet etching for certain process steps because plasma etching fails to provide the required etch selectivity, damage-free interface, and particle-contamination-free wafers. Using robotic handling and ultra-pure chemicals has improved particle control and process consistency. These improvements have revived wet etching processes for VLSI processing.
Two basic wet etching techniques are immersion etching and spray etching. Immersion etching is the simplest technique. The masked or unmasked wafer is submerged in the etch solution, and mechanical agitation is usually required to ensure etch uniformity and a consistent etch rate. Spray etching offers several advantages over immersion etching. Spray etching requires less volume of chemicals and is faster than immersion etching. Good process control and etch uniformity are easily obtained from spray etching because fresh etchant is constantly supplied to the wafer surface while the etch products are continuously removed. Single wafer spinning-chuck spray systems offer considerable process uniformity advantages.
FIG. 1 is a fragmentary sectional view of a spin etching device. As shown in FIG. 1, a semiconductor wafer 10 is horizontally placed on and fixed to a turntable 20 of a spin etching device. While the semiconductor wafer 10 is rotating, an etching solution 40 is supplied from a nozzle 30 disposed above the semiconductor wafer 10, dropping onto a substantially central area of the semiconductor wafer. The semiconductor wafer 10 is etched by the etching solution 40 as it is uniformly scattered over the semiconductor wafer under centrifugal forces.
The spin etching system as illustrated in FIG. 1 offers considerable process uniformity advantages. However, spin etching using viscous etchants such as sulfuric acid and phosphoric acid have generally failed to provide a uniform etch because the viscous etchants cannot spread uniformly across the wafer surface. The result is an incomplete etch or an undercutting of the surface layer, reducing the chip yield. The etch uniformity across the wafer becomes very important as film thickness is reduced, device dimensions shrink and wafer size increases. Therefore, an improved wet etching process that ensures a uniform etch is highly desirable.